4 Experimental 25
5.8 Blend surface morphology and phase separation
Figure 5.19: Microscope pictures taken with a magnification of 50x of a blends with a weight ratio of 1:1 (left) and 1:4 (right)
5.8 Blend surface morphology and phase separation
Blend morphology investigations via AFM were done only for F8T2:ICBA films with a donor-acceptor ratio of 1:1, because light microscopy showed large agglomerates in the 1:4 films, unsuitable for AFM. The topographical images can be seen in figure 5.20, where annealed and pristine blend of each polyfluorene are opposed to each other. The same was done for phase images as shown in figure 5.21.
Topography images:
The pristine P1 blend has a quite rough surface, where features have sizes of around one hundred nanometres. A refined topography is obtained for the annealed blend.
Features on the surface are easier to distinguish. P2 blends show amorphous behaviour, with large feature sizes of around 200 nm. The annealed blend shows no clear change of the film morphology. The character and the feature sizes seem to stay the same. This blend is therefore the most amorphous one (no reorganisation tendency). For pristine P3, a lamellar-looking structure appears where single features appear in an oval kind of shape. Annealing is completely changing the topographic structure. It seems to dissolve these fibre-like features. Instead round features appear, with sizes of 100 nm and below.
The images clearly show different surface morphologies for the blends fabricated from the three different polymers.
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Figure 5.20: AFM topography images taken from blends with a weight ratio of 1:1 (top, left: P1 not annealed; top, right: P1 annealed; middle, left: P2 not annealed; middle, right: P2 annealed; bottom, left: P3 not annealed;
bottom, right: P3 annealed)
60 Phase images:
Investigation of the phase composition of the untreated P1 film shows some kind of a branched structure of the polymer, with “dark” parts of ICBA that are distributed around the polymer (note: phase in AFM is induced by topographical features and different hardness of the surface; it is suggested that the darker regions represent the softer and more mobile material, therefore assigned to ICBA). The annealed blend exhibits definitely phase separated regions. It looks like the polymer forms round shaped features and ICBA filling up the free space in between. The image taken from the pristine P2 blend shows equally sized phase separations of foliate-like structure.
Also here, the dark parts around these structures indicate ICBA-rich areas. The features have sizes of about 200 nm. That changes for the annealed blend of the same polymer. The quite ordered structure was changed to an image where sharp peaks in the surface are visible. Due to that the picture is quite noisy. The phase images of both P3 blend are reflecting the topography images really well, as fibre-like phases of donor and acceptor material appear for pristine films. The annealed one shows smaller, round features of polymer that are distributed in no certain order. The fullerene derivative appears again as the dark areas.
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Figure 5.21: AFM phase images taken from blends with a weight ratio of 1:1 (top, left: P1 not annealed; top, right: P1 annealed; middle, left: P2 not annealed; middle, right: P2 annealed; bottom, left: P3 not annealed;
bottom, right: P3 annealed)
5.9 Molecular order and orientation
In figure 5.22 grazing incidence X-ray diffraction measurements of drop casted samples from all three polymers are shown. The x-axis is labelled with the in-plane scattering vector where the y-axis is labelled with the out-of-plane scattering vector.
Intensities are illustrated by a logarithmic colour code. It starts with blue for low intensities and goes to red for high ones. In figure 5.23 plots of the same measurements are shown, but in this case intensity over |q|. The absolute value of q was calculated by |𝑞| = 4𝜋𝜆 sin 𝜃, where λ=1 Å and θ is the scattering angle.
62
P1: In figure 5.22 plot (top, left) two rings are visible: a sharp one at 0.4 Å-1 and a broad one at 1.4 Å-1. These rings are characteristic for the separation of the side chains in F8T2 thin films. The smaller one indicates the distance of the polymer backbones with side chains in between. The other one shows the distance of the side backbones without side chains in between. Due to this ring having quite the same intensity for large and small 𝑞𝑧 values, there is no preferred orientation. The red dots in this plot display probably silicon dust. In figure 5.23 the same three features can be seen for P1. The ring at 0.4 shows a sharp, narrow peak in this illustration, with an intensity of about 4500 counts. The second ring is a quite broad feature with an intensity of 6500 counts. The surface contamination shows up as a small, kinky peak at 2.0 Å-1, which matches the value for Si (111).
P2: For P2 the measurements (figure 5.22, tor, right) show that the smaller ring vanishes and the larger one gets shifted to the right to 1.5 Å-1, but has with 4000 counts a lower intensity than for P1. This indicates more parallel stacking of the polymer backbones. What can be also seen in the measurement of P2 is that the larger ring has its highest intensity at large 𝑞𝑧 values. This shows a preferred polymer orientation dependence parallel to the substrate. The narrow, but distinct ring at about 2.1 Å-1 that appears in this plot is a signal from silicon that might come from breaking the substrate. It shows up as a very small peak in figure 5.23. What is also clearly visible is a distinct feature with high intensity at about 0.8 Å-1. This probably comes from residues from the synthesis and is definitely not a signal from an organic material.
P3: Figure 5.22 (bottom, left) shows no ring at 0.4 Å-1 as well, which means that the order in this arrangement is broken and parallel stacking of the backbone is preferred. The second ring shows with 3500 counts less intensity than for P2 and is according to figure 5.23 a little bit shifted back to the left to a value of 1.45 Å-1. This means less parallel stacking compared to P2 and more herringbone structure. As for P2 it has the highest intensity at large 𝑞𝑧 values. So the preferred polymer orientation is parallel to the substrate as well.
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Figure 5.22: Illustration of the in-plane over the out-of-plane scattering vector in a logarithmic colour code derived from drop casted polymer only films (P1: top, left; P2: top, right; P3: bottom, left)
With the positions of the diffraction rings, which are derived from the peaks in figure 5.23 the d-spacings d (spacing between adjacent lattice planes) can be calculated via:
2π
|q|= d (5)
The values can be found in table 5.3. The d-spacings detected for P1 are matching the results for X-ray diffraction under grazing incidence conditions for F8T2 thin films reported by Werzer et al. [44] very well. Due to Kinder et al. [45] the backbones of F8T2 lie in the plane of the substrate. So the d-spacing of about 16 Å is associated with the layering distance between sheets of polymer chains. This layering
64
happens because of the segregation of the backbones from the main chain from aggregated side chains. The smaller peak for d-spacings of around 4.5 Å could indicate the lateral distance between polymer chains in the formed layer. Another explanation could be the crystallization of the side chains.
Table 5.3: Detected absolute values q and calculated d-spacings
Polymer |q| (Å-1) d (Å)
P1 0.40
1.40
15.7 4.5
P2 1.50 4.2
P3 1.45 4.3
For P2 and P3 the d-spacing at around 4.5 Å is visible but the other one at around 16 Å disappears. That means that the order of the layering of the polymers backbones is getting lost for branched side chains.
Figure 5.23: Illustration of the intensity over the absolute value of the scattering vector derived from drop casted polymer only films
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5.10 Conclusions
For blends fabricated with three different types of F8T2 as donor material and ICBA as acceptor material (D/A weight ratio of 1:1 and 1:4) and sole polymer films, different electrical, optical and morphological investigations have been performed.
Photocurrent measurements revealed current densities for all material combinations with values up to -0.3 mA/cm2 and open circuit voltages between 0.8 and 1.0 V. The photoconversion efficiencies reached values of 0.1%. EQE measurements showed peak values of up to 0.5 %, and clear peak shifts, which reveal that blends fabricated from P1 produced the most crystalline and from P2 the least crystalline films. The emission dynamics showed triplet formation for polymer-only films as they exhibit slow decay times. For blends, efficient quenching and probably polaron formation were observed. Electroluminescence measurements indicated a charge transfer state arising at wavelengths of 800 nm and above. The recorded absorption spectra showed a maximum in the visible range at about 450 nm from the F8T2 and from the ICBA only a contribution in the ultraviolet range. Also here, peak shifts show that P1 films are the most ordered ones, while P2 films have the least order. AFM morphology investigations clearly revealed a phase separation between donor and acceptor material with domain sizes between 100 and 200 nm. The molecular order of polymer only films was investigated by grazing incidence X-ray diffraction. All three polymers showed signs of crystallization behaviour of the side chains, but only P1 exhibited a characteristic order in the stacking distance between sheets of polymer chains.
The device performances appear low, compared to similar reported material systems.
This might be due to a considerable concentration of palladium impurities in the polymers, a residue of the synthesis, where it was used as a catalyst. Nevertheless, a clear impact of the different ordered side chains on the ability to crystallization has been found, which reflects also in the photovoltaic performance.
66
PCBM = phenyl-C70-butyric acid methyl ester ITO = Indium tin oxide
IPA = Isopropyl alcohol
1,2 DCB = 1,2-dichlorobenzene VOC = open circuit voltage JSC = short circuit current density FF = fill factor
EQE = External quantum efficiency
A.2 List of illustrations
Figure 1.1: Illustration of the world’s total primary energy supply from 1850 to 2100 Figure 1.2: Diagram of the world’s energy consumption sorted by energy source for
the year 2006
Figure 1.3: Illustration of a polymer with single bonds (a) and a conjugated polymer (b)
Figure 1.4: Illustration of the chemical structure of the fullerene C60 (left) and the
67 fullerene derivative PC70BM (right)
Figure 1.5: Schematic illustration of a bilayer solar cell (left) and a BHJ solar cell (right)
Figure 1.6: Illustration of charge generation and separation processes taking place in an organic solar cell
Figure 2.1: Chemical structures (top row) and possible stacking structures (bottom row) of the three investigated polymers
Figure 2.2: Synthesis scheme for P1, P2 (top row) and P3 (bottom row) Figure 2.3: Illustration of the chemical structure of ICBA
Figure 2.4: Energy level diagram of F8T2, PC70BM and IC60BA
Figure 3.1: Schematic illustration of IV-curves in the dark and under illumination plotted linear (left) and semi-logarithmic (right)
Figure 3.2: Equivalent circuit diagram for an organic solar cell
Figure 3.3: Schematic illustration of the EQE, IQE and surface reflectance Figure 3.4: EL set-ups for spatially resolved measurements (left) and for
spectroscopic investigations
Figure 3.5: Schematic illustration of the set-up for TCSPC measurements Figure 3.6: Illustration of the Franck-Condon principle
Figure 3.7: Schematic set-up of an AFM Figure 3.8: Schematic illustration of GIXRD
Figure 4.1: Apperatures and processes used for the fabrication of solar cells: plasma etcher (a), spin coater (b), heating stage (c), applying of the active layer
Figure 4.5: Schematic illustration of the sample holder (left) and UV-VIS spectrometer by Shimadzu (right)
68
Figure 4.6: TCSPC set-up: detection chamber (left) and lens system (right) Figure 4.7: Illustration of the profilometer
Figure 4.8: Illustration of the AFM is the Nanosurf Easyscan 2 Figure 4.9: Illustration of the microscope BX52 from Olympus Figure 4.10: Schematic set-up for the X-ray measurements
Figure 5.1: Semi-logarithmic dark current curves, derived from blends with a D/A ratio of 1:1
Figure 5.2: Semi-logarithmic dark current curves, derived from blends with a D/A ratio of 1:4
Figure 5.3: Normalized EQE for blends with a D/A weight ratio of 1:1 Figure 5.4: Normalized EQE for blends with a D/A weight ratio of 1:4
Figure 5.5: Dark current corrected light IV-curves (AM 1.5G conditions) derived from blends with a weight ratio of 1:1
Figure 5.6: Dark current corrected light IV-curves (AM 1.5G conditions) derived from blends with a weight ratio of 1:4
Figure 5.7: Normalized EL emission for blends with D:A ratio of 1:1 Figure 5.8: Normalized EL emission for blends with D:A ratio of 1:4 Figure 5.9: Normalized PL emission derived from polymer only films
Figure 5.10: TCSPS lifetime decay curves of polymer only films, detected at 550 nm (top, left), 580 nm (top, right) and 640 nm (bottom, left)
Figure 5.11: PL emission curves derived from blends with a D/A weight ratio of 1:1 Figure 5.12: TCSPS lifetime decay curves of blends with D:A of 1:1, detected at
545 nm (left) and 735 nm (right)
Figure 5.13: Summarized decay times for polymer only films and blends with a weight ratio of 1:1
Figure 5.14: Absorption coefficients for annealed and not annealed blends with a weight ratio of 1:1
Figure 5.15: Absorption coefficients for annealed and not annealed blends with a weight ratio of 1:4
Figure 5.16: Normalized absorption of polymer only films
Figure 5.17: Normalized absorption of annealed and not annealed blends with a weight ratio of 1:1
69
Figure 5.18: Normalized absorption of annealed and not annealed blends with a weight ratio of 1:4
Figure 5.19: Microscope pictures taken with a magnification of 50x of a blends with a weight ratio of 1:1 (left) and 1:4 (right)
Figure 5.20: AFM topography images taken from blends with a weight ratio of 1:1 (top, left: P1 not annealed; top, right: P1 annealed; middle, left: P2 not annealed; middle, right: P2 annealed; bottom, left: P3 not annealed;
bottom, right: P3 annealed)
Figure 5.21: AFM phase images taken from blends with a weight ratio of 1:1 (top, left: P1 not annealed; top, right: P1 annealed; middle, left: P2 not annealed; middle, right: P2 annealed; bottom, left: P3 not annealed;
bottom, right: P3 annealed)
Figure 5.22: Illustration of the in-plane over the out-of-plane scattering vector in a logarithmic colour code derived from drop casted polymer only films Figure 5.23: Illustration of the intensity over the absolute value of the scattering
vector derived from drop casted polymer only films
A.3 List of tables
Table 2.1: Molecular weight and polydispersity of the F8T2’s
Table 4.1: Spin speeds and times for the used material combinations
Table 4.2: Evaporation rates and pressures when the evaporation was started
Table 5.1: Material combinations and according EQE values
Table 5.2: Material combinations and according fill factors and efficiencies Table 5.3: Detected absolute values q and calculated d-spacings
70
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